1. Field of the Invention
The present invention relates to an apparatus and method for producing purified hydrogen gas by a pressure swing adsorption process. More specifically, the present invention relates to detecting an operating life of adsorbents.
2. Description of Related Art
Pressure swing adsorption (PSA) processes are used for purifying hydrogen gas. In PSA processes, hydrogen gas included with impurity gases is fed to an adsorption tower filled with an adsorbent and the hydrogen gas passes through the adsorbent at a high pressure, while impurity gases, for example, N2, Ar, CH4 and chlorosilanes (dichlorosilane (abbreviated “DCS”), trichlorosilane, tetrachlorosilane (abbreviated “STC”), etc), are adsorbed by the adsorbent at high pressure. After a predetermined time has passed, a feed of unpurified hydrogen gas is turned off. The adsorption tower is depressurized by typically one or more steps, while a purified hydrogen gas is discharged from the adsorption tower. After that, the adsorbent is regenerated by depressurizing and purging with pure hydrogen gas and by heating up the adsorbent. A purified hydrogen gas is produced by repeating these steps, as described in U.S. Pat. No. 6,261,343 B1. Industrially, the PSA process includes multiple adsorption towers, with each tower connected to on-off valves and constantly produces a purified hydrogen gas, as in U.S. Publication No. 2007/0204748 A1.
Over time, the adsorption capacity of the adsorbents is reduced by repeating the PSA process. Finally, before the adsorbents are regenerated, impurities pass through the adsorbents without being adsorbed. This is know as the “breach” phenomenon.
PSA processes are used in many industry fields. Especially, the polycrystalline silicon manufacturing industry commonly uses PSA processes. Polycrystalline silicon is produced by feeding trichlorosilane (SiHCl3, abbreviated “TCS”) gas and purified hydrogen gas onto a surface of silicon seed rods at a high temperature in a reactor. This method is known as the Siemens method and is shown by following main reaction formulas:SiHCl3+H2→Si+3HCl(Si bulk surface)  (1)SiHCl3+HClSiCl4+H2(Gas phase)  (2)
In this method, exhausted gas from the reactor includes unreacted hydrogen gas. The unreacted hydrogen gas is purified by the PSA process and the purified hydrogen gas is reused as raw material gas for producing polycrystalline silicon.
In the polycrystalline silicon producing process, impurities are strictly controlled. It is important to detect an operating life of the adsorbents before a breach is caused. As a method for detecting a breach, Japan Publication Application Nos. S63-144110, H07-277720, and 2001-58118 propose methods in which a content of hydrogen chloride in the purified hydrogen is measured. But, lately, a method that more accurately detects impurity breach and an operating life of the adsorbents is needed.
One object of this present invention is to provide an apparatus and a method for detecting the operating life of adsorbents. Another object of this present invention is to provide an apparatus and a method for purifying a hydrogen gas for a polycrystalline silicon producing processes.